Comparative phylogeography of two colonial ascidians reveals contrasting invasion histories in North America
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Surveys of genetic structure of introduced populations of nonindigenous species may reveal the source(s) of introduction, the number of introduction events, and total inoculum size. Here we use the mitochondrial cytochrome c oxidase subunit 1 (COI) gene to explore genetic structure and contrast invasion histories of two ecologically similar and highly invasive colonial ascidians, the golden star tunicate Botryllus schlosseri and the violet tunicate Botrylloides violaceus, in their global and introduced North American ranges. Haplotype and nucleotide diversities for B. schlosseri were significantly higher than for B. violaceus both globally (h = 0.872; π = 0.054 and h = 0.461; π = 0.007, respectively) and in their overlapping North American ranges (h = 0.874; π = 0.012 and h = 0.384; π = 0.006, respectively). Comparative population genetics and phylogenetic analyses revealed clear differences in patterns of invasion for these two species. B. schlosseri populations on the west and east coasts of North America were seeded from the Pacific and Mediterranean regions, respectively, whereas all North American B. violaceus populations were founded by one or more introduction events from Japan. Differences in genetic structure of invasive populations for these species in North America are consistent with their contrasting probable introduction vectors. B. schlosseri invasions most likely resulted from vessel hull fouling, whereas B. violaceus was likely introduced as a ‘fellow traveler’ in the shellfish aquaculture trade.
KeywordsNonindigenous species Mitochondrial DNA Introduction Tunicates Botryllus schlosseri Botrylloides violaceus
We acknowledge our Canadian Aquatic Invasive Species Network (CAISN) colleagues G. Arsenault, J. Davidson, J. Hill, and A. Ramsay as well as our global colleagues T. Bolton, P. Chevaldonné, R. Graille, E. Grey, A. Izquierdo-Munoz, B. de Ligondes, S. Lopez-Legentil, J.-M. Nicolas, M. Ruis-Viladomiu, Y. Saito, X. Turon, and B. Vercaemer, who have so generously provided tunicate samples. D. Heath, S. Xu and A. Zhan provided valuable comments on an early draft of the manuscript. We also thank A. Adebayo, K. Laroche and R. Hepburn for assisting with lab work. This work was supported by CAISN, Fisheries and Oceans Canada and by NSERC Discovery grants to MEC and HJM.
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